Modern copiers and facsimile machines feature more compact designs than such devices heretofore. The compactness requirement in such machines raises packaging problems including problems of heat dissipation. More particularly, the limited available space in the vicinity of the document scanner glass contributes to the need for innovative and energy efficient illumination system.
On the one hand, it is desirable to use a fluorescent light source because it is energy efficient and operates at a low temperature. On the other hand, the length and bulkiness of a fluorescent lamp contribute to a configuration problem because of the space-limited environment typically encountered in such compact machines. A design solution is to place the bulky fluorescent lamp at a suitable position away from the document scanner glass, where space or heat dissipation may be less of a problem, and guide or "pipe" the light to the congested document scanner glass area using a light guide.
In order to provide sufficient illumination to a target document area, multiple sources of illumination having multiple light guides may be needed, or an optimized illumination system having an efficient light guide may be required. However, the use of multiple light sources and the associated light guides needed tends to contribute to the aforesaid space congestion problem. An optimized illumination system requiring only one efficient light guide and a single high intensity light source, which provides adequate illumination to the target document area, would be a far more cost-effective realization.
Some prior illumination systems provide such multiple light sources having multiple light guides. For instance, U.S. Pat. No. 3,947,106 to Hamaguchi et al discloses an illumination package for a copier having two light guides and two associated light sources for providing illumination to the document area to be scanned.
The described light guides have output windows directed to the document area, and input windows, which are wider than their respective output windows, each adjacent its fluorescent light source, respectively. This choice of the light guide orientation, contrary to intuition, does not produce light concentration or deliver more light. In fact, because of the wide input window, this orientation requires a fluorescent lamp with a large aperture angle, which tends to reduce source radiance and results in a reduction in overall document area irradiance.
In addition, employment of a reflective coating on the outside walls of light guide, as described, also contributes to reflection and transmission losses.
A prior illumination device employing a single light guide for use in photocopiers is described in U.S. Pat. No. 4,128,332 to Towe. It employs an internal reflecting light conduit and a light source to illuminate at least one of two input ends of the light conduit. The longitudinal surface of the light conduit has a coating arranged in desired geometry to allow the escape of light from the conduit in accordance with the geometry to illuminate a desired document area. There is no teaching of either the use of an apertured light source having a critical aperture angle or means to maximize document illuminance.
Another illumination device for document scanning is described in U.S. Pat. No. 4,186,431 to Engel. It employs a cylindrical elongated incandescent lamp positioned within a tubular light reflecting member having a line light diffuser strip window. A narrow beam of light emitted through the diffuser strip window is projected onto the document by a cylindrical lens. There is no teaching of a light conducting means having an output window at one end and a relatively narrower input window at the other end and being oriented so that the narrower input window is adjacent the aperture of the light source and the output window is directed to the document area to be scanned, whereby the document irradiance is increased.